When measuring milk quantities on the farm or in the dairy the mass of the milk is of interest. For this reason the weighing of the milk would be the right method for exactly determining the mass. However, an exact weighing of the milk is technically very difficult in the rough everday use in the stables on a farm, in particular, if only small, possibly mobile milk quantity measuring devices should to be used.
In such weighing devices the influences of force of the connected feed and discharge tubes, the kinetic energy of the obtained milk pulsating, vibrations due to careless handling, a non-horizontal measuring base, cleaning problems in internal measuring chambers with the necessary pressure compensation lines, etc. would entail considerable problems and also measuring errors. Therefore it is not astonishing that it was impossible for weighing milk quantity measuring systems to be successful in rural everyday practice outside of research institutes.
Practically all milk quantity measuring devices known at present attempt to ascertain the mass of the obtained milk by means of a volume measurement. Devices have become known which comprise for instance milk quantity measuring vessels firmly installed at the milking station or devices in which part of the milk flow is in each case put aside and measured. Socalled flow meters are also already known which work either intermittently batchwise or carry out a continuous quantity measurement.
An example of the first type of measurement is known from the DE-OS 30 05 489, in which the obtained milk is guided into a large storage vessel. A measuring probe is disposed in the storage vessel which comprises several measuring electrodes at the same height distance above each other which cooperate with a joint stationary counter-electrode. In this measuring process the property of the milk that is used is that it has a relatively great conductivity with respect to the insulator air to determine at different electrodes by means of the cyclic application of an electric field which electrode circuits are already closed due to the level of the milk. Thus each electrode circuits is interrogated regarding a yes/no decision in the sense that the electrode circuits located below the milk level give the information yes-closed, while the electrode circuits located above the milk level indicate the information no-not closed.
A process for the determination of the portions of three different fluids, namely water, crude oil and gas is known from the U.S. Pat. No. 3,530,711 in a completely different field, namely in the field of crude oil drillings. Electrodes being disposed in vertical staggered relationship with respect to each other are provided with a joint counter-electrode in a measuring pipe in which the sample taken can settle, whereby three separate layers and thus two boundary layers result due to the different density. The electrodes are cyclically interrogated with an a-c voltage and the capacity is measured in each case. Due to the different dielectric constants of water, crude oil and gas, the position of the respective boundary layers and thus the size of the respective volume can be determined at the boundary layers due to a jump in the size of the measured capacitance.
A process for the quantity determination of two galvanic liquids (liquids metals) stacked above each other with different conductivity has also already been known from the U.S. Pat. No. 3,370,466. Several pairs of electrodes are disposed in a measuring vessel in each case at the same height distance, to which a d-c voltage is cyclically applied. The liquid limit to be determined is between successive pairs of electrodes between which a jump in the electrical conductivity is determined.
A measuring system for measuring the liquid height in a cylinder is also already known from the U.S. Pat. No. 4,450,722, in which red/green light sources and red/green light sensors are disposed in each case at the same height distances above each other at opposite sides of the transparent measuring cylinder. The red portion of the light is absorbed in such measuring paths which are below the water level so that only the green sensors respond and produce a green signal. On the other hand both a green and a red signal is produced in such measuring paths which are above this height level of the water level. The occurrence or disappearance of the red signal shows indicates that there must be the water level between these height levels.
From the DE-OS 16 07 007 and 16 32 938 devices for milk quantity measurement area also already known, in which the obtained milk is in each case sprayed in one jet vertically from below against a concave baffle screen so that a milk liquid screen reaching across 360.degree. results. The milk flowing off across a certain angular range of this screen is collected and supplied to a measuring cylinder. The height of the milk level in this measuring cylinder is read with the eye and represents practically the share of the total amount of milk which corresponds to the ratio of the angular range on which the milk is collected to 360.degree..
From the GB-PS 1 316 573, the DE-AS 28 10 376 and the EP 0 057 267 milk quantity measuring devices are also already known which measure intermittently batchwise. The obtained milk is introduced into a measuring vessel until a floater floating on the milk or a sensor located at a predetermined height emits a signal due to the milk level reaching this sensor. Then the supply of further milk is interrupted while at the same time an overflow valve is opened at the measuring vessel from which the entire milk can flow off or the milk can flow off as long as a sensor located at a lower level emits a further signal whereupon the milk overflow valve is closed again and the inflow of obtained milk accumulated in the meantime is released into the measuring vessel. Since the volume between the sensor located at the upper height level or the second senor located at the lower height level is known, the total quantity of milk can be determined by adding up the individual batches assuming that the milk has a constant specific density.
A milk flow meter is also already known from the DE-OS 32 10 465, in which the obtained milk is introduced into a retaining vessel which has an outlet with a predetermined cross-section at its lower end. In this arrangement the retaining height of the milk is to be detected capacitively by means of electrodes disposed at the inner walls of the retaining vessel to determine the respective milk flow of the milk flowing off in connection with the cross section of the outlet line.
A similar device of a milk flow meter is also already known from the U.S. Pat. No. 4,452,176 in which the obtained milk also flows into a retaining vessel from which it flows off via a vertical measuring slot. Also here the retaining height is to be determined from a capacitance measurement to determine the milk flow. A problem common to both devices resides in that due to the mixing of the milk with air the retaining height cannot be unequivocally determined by means of a capacitive measurement and, on the other hand, the milk flown off via the outlet cross-section can likewise not be unequivocally determined, because the specific density of the milk flowing off is changed in broad limits.
The problem of milk quantity measurement which has so far only insufficiently been taken into account resides in that milk is a very foaming fluid so that in the case of the volume measuring processing the measurement is falsified by the foam or the air portion so that the mass can no longer be unequivocally inferred from the volume as this is for instance possible to a very great degree in the case of water.
First of all milk gases, in particular carbon dioxide and nitrogen, are bonded in the milk which vary approximately between three and nine per cent by volume. The by far still greater gas portion is however caused by the admixing of air, in particular during the milking process. An air/milk mixture is produced in the milking machine for transporting the milk through an air inlet in the milking equipment, which contains approximately between 30 and 1 per cent by volume of milk depending upon the milking conditions. In order to eliminate the air from the milk a degassing path or a degassing vessel with less turbulence and sufficient surface is needed. While large gas bubbles, approximately with a diameter of 3 mm, rise relatively rapidly, with an end speed of approximately 300 mm/s, small bubbles, e.g. with a diameter of 0.3 mm take approximately ten times longer. The measuring problem in the volume measurement of milk is thus mainly caused by the small milk bubbles which account for approximately 10 to 15% of the milk volume.
These small bubbles cannot be satisfactorily removed from the milk with mechanical separating means such as inlet cyclone, filling of the measuring chamber from below, etc. in particular not in small milk quantity measuring devices suited for the mobile use on the farm with correspondingly short milk residence times.
The air portion and the bubble size are however not always the same, but depend on a plurality of factors which cause a different foam formation. Such factors are for instance the milk flow quantity, the guiding of the milk tube, the type of the milking system, the type of teat rubber, the diameter of the milking tube, the type of the milking system, the feeding of the cows which changes the milk composition, the health of the udders, the differences between the individual cows and differences in each individual cow due to the lactation phase.
Due to these given factors it is practically impossible to estimate for instance a height in a milk vessel, of which it is assumed that the volume below this height consists of pure milk, while the foam located above this height is neglected as no longer containing any essential milk share. That is to say that the attempt of getting the foam share under control by a corresponding general calibration of the level measurement is doomed to failure, in particular in small vessels as they are above all necessary for mobile milk quantity meters. The air portion of the accumulated fluid is not rarely 30 per cent by volume and more in such vessel sizes. And even in large-volume milk quantity measuring devices, socalled recorders, the white horse may contain between 0 and 0.5 kg depending upon foam height and foam consistency which can account for approx. 5% in a typical milking output of for instance 10 kg per milking process. Recorders are customarily read at the boundary layer milk/foam, i.e. one does not evaluate the amount of milk in the foam because one does not know the foam consistency.
A device for the monitoring or measurement of the foam level in a flotation concentration system is already known from the DE-OS 27 20 006. Several electrode rods are provided in this arrangement which are disposed in vertical arrangement in parallel to each other and project with their lower end against the surface of the liquid to a differently great degree. As soon as the foam contacts an electrode upon a rising of the foam, an electrical circuit is closed due to the conductivity of the foam. Thus the height of the foam is on the whole indicated by the number of the closed circuits of the individual electrodes. The height indication is also effected here due to a yes/no indication of the individual electrode circuits.
Above reference was made in each case to milk as a foaming liquid. However, all other foaming liquids such as in particular beer or fruit juices or other technical foaming liquids such as galvanizing liquids have the same problems as milk.
The present invention is based on the object to determine the foam profile of a foaming liquid, i.e. to determine the specific density of the liquid/air mixture as a function of the height.
This is achieved according to the invention starting from a measuring process of the type mentioned at the beginning by measuring a reference measuring value (I.sub.O) for the measurement of the specific density of the foaming liquid at different height levels on a reference measuring path containing substantially degassed liquid, that as a function of the fact whether a measuring value (I.sub.L) measured in air is greater or smaller than the reference measuring value (I.sub.O) obtained on the reference measuring path a ratio value (c.sub.m) according to the ratio from the reference measuring value (I.sub.O) and the measuring value at this height level (I.sub.m) or the reciprocal value of this ratio is formed for each height level, that possibly in accordance with a preceding calibration a corrected ratio figure (c'.sub.m) which is equal to 1 for the deaerated liquid and substantially equal to zero for air is formed and that each ratio value (c.sub.m, c'.sub.m) is multiplied by the value for the specific density (.rho.) of the degassed liquid.
The process according to the invention provides the prerequisite of determining the mass of a foaming liquid due to a volume measurement by being able to determine the share of the respective liquid at each height of the liquid/air mixture. It can be achieved that by a suited selection of the parameters of the measuring device the measured ratio values c.sub.m are equal to the desired facts, which indicate the specific density at the respective height level by a multiplication by .rho.. Possibly a calibration according to a process indicated below must be carried out once in order to thus obtain corrected ratio values c'.sub.m by means of a correction.
Advantageously the mass of the liquid contained in a vessel can be determined by determining the ratio value (c'.sub.m) at each height level (m), that the volume (V.sub.m) being located between a height level and the next lower height level or the bottom of the vessel is determined in each case, that in each case the product (c'.sub.m .times.V.sub.m) from the volume (V.sub.m) located below a height level, the ratio value (c'.sub.m) determined for this height level and the specific density of the milk (.rho.) is formed in each case and that for determining the entire liquid mass (G) the sum of all products thus formed is formed across all height levels (n) in accordance with ##EQU1##
Thus a process is provided according to the invention in which the entire measuring volume is subdivided into layers and in which a specific density of the milk/air mixture is determined for each layer by measuring the ratio value, which represents the instantaneously present milk/air ratio. Thus the milk mass contained in the foam can for the first time be detected in a volume measurement and taken into account in the determination of the total milk mass.
Starting from the aforementioned equation for the total mass it becomes readily apparent that under certain prerequisites the formation and processing of the individual measuring values can also be carried out in another fashion to reduce the necessary time for each total measurement. If one proceeds from the assumption that the volume V.sub.m is constant=V.sub.O at each height level and that each calibrated ratio value c'.sub.m is represented by ##EQU2## wherein I'.sub.m means the calibrated measuring value for the height level m and I'.sub.O means the calibrated reference value, then the aforementioned equation can be simplified to ##EQU3## is constant, the measurement would be reduced to an adding up of the calibrated measuring values I'.sub.m and a multiplication by the factor ##EQU4##
If, on the other hand, one considers that n. is the total volume V across all height levels and that ##EQU5## can be considered as a calibrated ratio value c' and can be averaged by means of via n, the mass G can be determined from EQU G=V.multidot..rho..multidot.c'.
It is evident that also in this case the ratio ##EQU6## must not be formed at first in each case, but that at first the sum ##EQU7## may be formed.
In order to simplify the measurement as described above by selecting an equal volume V.sub.O at each height level, a cylindrical vessel with optional base area is preferably used and the height levels are provided at equal mutual height distances. However, the same volumes V.sub.O could of course also be achieved with irregular vessel cross-sections if the electrodes are disposed in corresponding different height distances which are adapted to the cross-sectional shape.
The reference measurement should be carried out in the same milk which is also collected in the actual measurement to avoid that there are any differences due to another milk consistency, etc. This reference measurement can be carried outside the actual milk vessel and it should only be ensured that the milk is deaerated to a very large degree, that is to say, that it practically no longer contains any air bubbles. Since however a reference measuring path outside the milk measuring vessel again renders the measurement altogether more difficult, the reference measurement is preferably carried out at the bottom of the vessel itself. It is proceeded from the experience here that in the case of the measurements in question milk has already accumulated up to a certain height before the measurement is carried out. Under these conditions the milk located near the bottom is already largely deaerated in the case of a suited dimensioning of the vessel.
It became apparent that the measurements can fundamentally be carried out with different methods using different parameters of the milk. Such measurements are especially suited for this, in which the measuring value which results for deaerated milk differs by at least one magnitude from the measuring value measured for air. The ratio value is then formed from these values in such fashion that for the ratio of the measuring value for air in relation to the reference measuring value or by formation of the reciprocal value a ratio value substantially smaller than 1 results, while for the ratio of the measuring value for the deaerated milk in relation to the reference measuring value the value 1 results in each case automatically.
Measurements of this type can be carried out using the properties of the milk such as the electrical conductivity, the thermal conductivity or the infrared absorbing power, which vary very strongly as a function of the milk/air ratio. The resistance of the measuring path can serve as a measuring magnitude using the change of the electrical conductivity of the milk, the amount of light transmitted can serve as a measuring magnitude using the IR absorption or the voltage drop at a temperature sensor can serve as a measuring magnitude using the thermal conductivity of the milk.
According to a preferred embodiment of the invention deviations of the individual measuring paths resulting due to changes or soilings of the individual electrodes or measuring paths can be compensated by carrying out the same measurements at all height levels using the same calibration liquid such as water. A mean value is formed from the resultant measurements including the measurement on the reference measuring path and the deviations of the individual measuring paths from this mean value are taken into account with a corresponding correction factor for the actual measurement.
It is possible to select the parameters of the measuring device suitably in such fashion that the measured ratio figure c.sub.m does no longer require any correction. However, in general it is necessary to calibrate a construction type of measuring device once before the actual measurements. Due to this the actually measured ratio value c.sub.m is corrected in accordance with the specific density of the milk/air ratio. As became apparent this can be effected in a simple case by exponentiating, that is, raising to a higher power, the measured ratio values c.sub.m in each case with an exponent greater than zero to form corrected ratio values c'.sub.m. If the exponent in such a case has been determined once by a calibration it remains unchanged for all later measurements.
According to the invention a process for the measurement of the flow of a foaming liquid, in particular for the measurement of the flow of milk mixed with air is indicated, in which a measuring value depending on the same parameter of the liquid contained in a vessel is measured in each case at several different height levels and which distinguishes itself by the fact that liquid is supplied to the vessel, that liquid flows off continuously via a substantially vertical measuring slot, that a reference measuring value (I.sub.O) is measured on a reference measuring path containing substantially degassed liquid, that as a function of the fact whether the measuring value (I.sub.L) measured across a corresponding measuring path in air is greater or smaller than the reference measuring value, a ratio value (c.sub.m) in accordance with the ratio of reference measuring value (I.sub.O) to the measuring value at the respective height level (I.sub.m) or in accordance with the reciprocal value of this ratio is formed for each height level (m) and that the quantity of liquid flowing off through the measuring slot per time unit is determined from the equation ##EQU8## K=d.times.s.times..rho..sqroot.2 gd d [cm]=distance of electrodes=distance of height levels ##EQU9## s [cm]=slot width ##EQU10## n=total number of electrodes c'.sub.m =formed ratio figure between 1 and 0 at the height level m
a=constant of the measuring device depending on slot width, slot edge, etc., which can be ascertained by calibration.
The process can also be used to determine the total mass of the flown liquid by a subsequent summation or integration of all measured flows.
The formula results by a derivation from the socalled Bernoilli's equation by calculating the outflow rate of the liquid/air mixture resulting due to the hydrostatic pressure in a height level for each height of the slot taking a vertical slot as a basis for each height of the slot and a customary correction for the outflow behaviour of a fluid at a slot is taken in account as a function of the speed, with which the hydrostatic pressure at a certain height level can be computed from the measured foam profile and the specific density at this height level is also determined by means of a measurement. The flowing out at a vertical slot represents of course only a special case which is not to restrict the inventive idea. The flow of a foaming liquid can likewise be computed for instance also by a simple computation, which flows off for instance through an opening provided at the bottom of the vessel since the hydrostatic pressure of this liquid at the flow outlet can be determined by measurements of the respective liquid portion at the different heights.
According to the invention a device for the measurement of the specific density of a foaming liquid, in particular of a milk/air mixture is indicated comprising a vessel and at least one measuring device with which a measuring value can be measured in each case depending on the same parameter of the liquid contained in the vessel at several different height levels of the vessel, which is distinguished by the fact that a reference measuring path containing substantially deaerated liquid is provided, that a device is provided which forms a ratio value (c.sub.m) corresponding to the ratio from the reference measuring value and the measuring value at this height level or in accordance with the reciprocal value of this ratio for each height level as a function of the fact whether a corresponding measuring value (I.sub.L) measured in air is greater or smaller than the reference measuring value (I.sub.O) obtained on the reference measuring path, that a corrected ratio figure (c'.sub.m) which is equal to 1 for the degassed liquid and substantially equal to zero for air is possibly formed in the device in accordance with a preceding calibration and that a multiplication element is provided with which each ratio value (c.sub.m ; c'.sub.m) is multiplied by the value for the specific density (.rho.) of the degassed liquid.
Such a device can be suitably used in a device for measuring the liquid quantity, which is distinguished by the fact that a computing means (MP) is provided which multiplies the ratio figure (c'.sub.m) determined for each height level (m) by the size of the volume (V.sub.m) in the vessel enclosed between this height level and the height level located thereunder and the specific density of the degassed liquid (.rho.) so that the product c'.sub.m .times.V.sub.m .times..rho. is formed and that an adding means is provided for adding the products formed for all height levels to indicate the total quantity of liquid (G) as ##EQU11##
For the measurement of milk a device has proved to be especially suited, in which an electrode is disposed at the vessel at each height level and a joint counter-electrode facing all electrodes is provided. Using the change of the electrical conductivity of the milk as a function of the milk/air mixture the electric resistance on each measuring path, i.e. between an electrode and the counter-electrode is preferably measured.
For this purpose an a-c voltage is preferably used to avoid polarisations. Moreover a decoupling capacitor is suitably switched between the voltage source and the joint counter-electrode to eliminate any d-c portion. The frequency should preferably be between 200 Hz and 80 kHz and more preferably be 2 kHz to improve the switching on behaviour and to avoid time-dependent drift phenomena.
In view of the air bubble size occurring with preference in milk, electrodes are used here which are substantially circular and have a diameter ranging from about 0.5 to 1.2 mm. In fine optimizing a stronger dependence on small air bubbles was detected for the larger diameter of this range and a stronger dependence on large air bubbles was detected for small electrodes of this range. In order to achieve a dependence being as uniform as possible, an electrode diameter of 0.8 mm is preferably used.
The mutual height distance of the electrodes from each other was preferably in a range of 1 to 8 mm. Especially advantageous results were achieved at a height distance of 1.5 mm. The smaller the distance between electrode and counter-electrode was, the stronger was the change of the ratio value as a function of the respective measuring value. Therefore electrode distances between 2 and 150 mm and more preferably between 3 to 8 mm were used.
In the embodiments in which the ratio values are formed from measuring values of the electrical conductivity of milk it became apparent that the corrections of the measured ratio values necessary due to a calibration could be achieved by exponentiating with the same figure greater than zero.
According to a further preferably used quantity measuring device an IR light source and a mirror arrangement are provided by means of which the IR light ray can be radiated successively at different height levels through the milk contained in the vessel and an electrooptical transducer common to all height levels or an electrooptical transducer for each height level are provided which generates an electrical measuring value signal corresponding to the received luminous intensity.
A further quantity measuring device using the change of the thermal conductivity of a milk/air mixture distinguishes itself by PTC temperature sensors disposed at the milk vessel at different height levels, by constant-current sources, which supply in each case a constant heating capacity to the PTC temperature sensors and by resistance measuring circuits which determine the resistance value corresponding to the temperature of a PTC temperature sensor as a measuring value.